[1] [1] WANG D, XI L, TANG X, et al. A simple photonic precoding-less scheme for vector millimeter-wave signal generation based on a single phase modulator[J]. Results in physics, 2020, 19(103412):1-5.

[2] [2] WEI X, MU H, LI M, et al. Arbitrary free spectral range control of optical frequency combs based on an optical tapped delay line structure cascaded with a phase modulator[J]. Optoelectronics letters, 2021, 17(7):0390-0394.

[3] [3] CUI Y, WANG Z, XU Y, et al. Generation of flat optical frequency comb using cascaded PMs with combined harmonics[J]. IEEE photonics technology letters, 2022,34(9):490-493.

[4] [4] GUO Y, WANG M, MU H, et al. Simultaneous frequency and bandwidth doubling linearly chirped waveform generation based on cascaded phase modulator and dual-output dual-parallel Mach-Zehnder modulator[J]. Optik, 2021, 243(19):167384.

[5] [5] ULLAH S, ULLAH R, ZHANG Q, et al. Ultra-wide and flattened optical frequency comb generation based on cascaded phase modulator and LiNbO3-MZM offering terahertz bandwidth[J]. IEEE access, 2020, 8: 76692-76698.

[6] [6] SUMIDA M. Optical time domain reflectometry using an M-ary FSK probe and coherent detection[J]. Journal of lightwave technology, 1996, 14(11):2483-2491.

[7] [7] IIDA H, KOSHIKIYA Y, ITO F, et al. High-sensitivity coherent optical time domain reflectometry employing frequency-division multiplexing[J]. Journal of lightwave technology, 2012, 30(8):1121-1126.

[8] [8] LU L, SONG Y, FAN Z, et al. Dual frequency probe based coherent optical time domain reflectometry[J]. Optics communications, 2012, 285(10):2492-2495.

[9] [9] LU L, SONG Y, FAN Z, et al. Coherent optical time domain reflectometry using three frequency multiplexing probe[J]. Optics and lasers in engineering, 2012, 50(12):1735-1739.

[10] [10] LU L, SONG Y, ZHANG X, et al. Frequency division multiplexing OTDR with fast signal processing[J]. Optics & laser technology, 2012, 44(7):2206-2209.

[11] [11] ZHOU J, PAN Z, YE Q, et al. Characteristics and explanations of interference fading of a phi-OTDR with a multi-frequency source[J]. Journal of lightwave technology, 2013, 31(17):2947-2954.

[12] [12] QIAN H, LUO B, HE H, et al. Fading-free φ-OTDR with multi-frequency decomposition[J]. IEEE sensors journal, 2022, 22(3):2160-2166.

[13] [13] XU N, WANG P, WANG Y, et al. Crosstalk noise suppressed for multi-frequency φ-OTDR using compressed sensing[J]. Journal of lightwave technology, 2021, 39(22):7343-7349.

[14] [14] LV L, SONG Y, ZHU F, et al. Performance limit of a multi-frequency probe based coherent optical time domain reflectometry caused by nonlinear effects[J]. Chinese optics letters, 2012, 10(4):4.

[15] [15] LU L, SUN X, BU X, et al. Coherent optical time domain reflectometry by logarithmic detection and timed random frequency hopping[J]. Optical engineering, 2017, 56(2):024106.

[16] [16] WANG Z, WU Y, XIONG J, et al. Bipolar-coding φ-OTDR with interference fading elimination and frequency drift compensation[J]. Journal of lightwave technology, 2020, 38(21):6121-6128.

[17] [17] HAN M, WANG A. Analysis of a loss-compensated recirculating delayed self-heterodyne interferometer for laser linewidth measurement[J]. Applied physics B, 2005, 81(1):53-58.

[18] [18] CHEN M, ZHOU M, WANG J, et al. Ultra-narrow linewidth measurement based on Voigt profile fitting[J]. Optics express, 2015, 23(5):6803-6808.